Information recording media are widely researched, developed and commercialized. The information recording media allow information to be recorded thereon and erased therefrom when a thin film thereof formed of, for example, a chalcogen material, is irradiated with a laser beam and thereby is heated locally so that the difference in irradiation conditions causes a phase change between states with different optical constants.
Among the information recording media that utilize the phase change, an optical recording medium allows signals to be recorded through the modulation of output power of a laser beam between at least two power levels. Suitable selection of the power level makes it possible to record a new signal simultaneously while erasing a signal that has been recorded.
Research and development have been conducted actively for optical information recording media that allow information to be recorded thereon, and reproduced and erased therefrom, which are formed by stacking a plurality of component layers (hereinafter referred to as information layers) including a phase-change recording film whose phase changes by a laser beam that is incident from one side. For example, in a commercialized optical information recording medium having two information layers that allow information to be recorded thereon and reproduced therefrom, recording and reproducing with respect to one of the information layers (hereinafter referred to as a second information layer) disposed farther from the laser beam incident side are performed with a laser beam that has been transmitted through the other information layer (hereinafter referred to as a first information layer) disposed closer to the laser beam incident side. Accordingly, it is preferable that the first information layer has as high a transmittance as possible.
As one method for achieving the high transmittance, it is proposed to use, for a material layer (transmittance adjusting layer) constituting the first information layer, a material that is transparent and has a high refractive index with respect to the wavelength of the laser beam to be used. For example, it is proposed to use TiO2 for the transmittance adjusting layer in a recently-disclosed dual-layer rewritable optical recording medium using a blue laser (see WO 03/025922).
Also, as the material for a protective layer that directly covers a recording layer, a material has been disclosed that contains TiO2 as a main component, and 2.5 wt % to 11 wt % of Nb2O5, which is almost equal to 4 mol % or less of Nb2O5, or 3 atom % or less of Nb atoms, both added thereinto (see JP 2003-013201 A).
However, TiO2, which has been used conventionally as a material for the transmittance adjusting layer, has problems in that it takes a longer time to manufacture an information recording medium since it has a low deposition rate during sputtering and a great variation in deposition rate due to moisture, and it is difficult to maintain a constant thickness of the TiO2 film.
With the material that contains TiO2 as a main component and Nb2O5 added thereinto, it is difficult to increase the refractive index sufficiently. When the material is used for the transmittance adjusting layer of the first information layer, it is difficult to increase the transmittance of the first information layer